The measurement of open channel flow in municipal wastewater collection systems is important to protect public health, municipal infrastructure, and the environment. Raw (untreated) drinking water, irrigation water, and plant effluent water are also transported via engineered open channels and pose similar measurement challenges. Accurate flow metering is necessary for billing, engineering studies, mitigation of unwanted inflow and infiltration, and for the control of the actual flow itself. Flow volumes must be understood and managed to minimize the impact of peak flows on wastewater treatment facilities and to reduce the possibility of untreated sewage reaching the environment.
There are a number of open channel flow meters in existence today. For example, there is a flow meter for measuring both the fluid velocity and the fluid level by non-invasive level sensors and velocity sensors that can be mounted inside a manhole above the flowing channel (typically just above the top of the pipe or culvert, a crown mount). The velocity and level signals can be combined with knowledge of the pipe geometry, and by using a surface velocity modifier, the instrument electronics converts the sensed surface velocity to approximate the cross-sectional average velocity of the stream.
By way of example, a carrier frequency signal may be directed toward the surface of a fluid flowing in an open channel. More particularly, the signal is directed along a line toward the fluid surface and at least a portion of the signal is reflected from the fluid surface, and the Doppler frequency shift between the directed and reflected signals is used as a measure of the velocity of the fluid surface. Typically, a non-invasive measure of the fluid depth is also obtained, e.g., using downward-looking ultrasonic or other measurement techniques. From the measurements of velocity and depth, the flow rate of the fluid is calculated.